The invention relates to a method and apparatus for surveying positions of mooring lines extending between a moored structure and anchors in the bottom of a body of water.
The current use in deep water areas of permanently moored structures such as floating platforms carrying drilling or oil and gas production facilities has introduced safety problems associated with the positioning of mooring lines in relation to underwater facilities, such as well heads and flow lines on the water bottom. Detailed instructions for the deployment and operation of such moorings exist which permit satisfactory theoretical mooring patterns to be devised to minimize accidents due to anchor dragging and so forth. However, during the entire anchoring period measurements must still be carried out to check whether the touch points of the mooring lines onto the water bottom and the clearance between the mooring lines and the underwater installations meet the operating requirements. Such measurements, required under both long-term static conditions and during short-term movements of the vessel, are currently carried out by calculation of the mooring line catenary using measured values of the mooring line tension. The directions of the mooring lines in relation to the moored structure are measured when the structure is first positioned. It is thereby assumed that the submarine mooring lines take up a straight line between the structure and anchor. These techniques have been found to be inaccurate.
The primary object of the present invention is to provide a mooring line surveying method and means for continuously monitoring mooring line positions during extended periods of time without affecting the operations performed on the moored structure and, in particular, to provide a method and means for detecting the location of each of the touch points of the mooring lines onto the water bottom with an accuracy considerably superior to that of the techniques currently employed. A further object of the present invention is to provide a surveying method that is able to respond quickly to changes in mooring line positions, for instance, effects caused by oscillating movements of the moored structure due to wind, waves, tides and currents.